Veneer ceramic for dental restorations and method for veneering dental restorations

ABSTRACT

The invention is directed to veneer ceramics for dental restorations of framework ceramics comprising yttrium-stabilized zirconium dioxide. It is the object of the invention to make possible a translucent veneer ceramic which has high flexural strength as well as excellent adhesion to the framework ceramic of yttrium-stabilized zirconium dioxide. According to the invention, this object is met in a veneer ceramic for dental restorations made of yttrium-stabilized zirconium dioxide which is produced from the following components:
         a) SiO 2  58.0-74.0 percent by weight   b) Al 2 O 3  4.0-19.0 percent by weight   c) Li 2 O 5.0-17.0 percent by weight   d) Na 2 O 4.0-12.0 percent by weight   e) ZrO 2  0.5-6.0 percent by weight.

This present application is a continuation application of U.S.application Ser. No. 12/530,595, filed Oct. 20, 2010, which is the U.S.National Stage filing under 35 U.S.C. §371 of International ApplicationNo. PCT/DE2008/000405 filed Mar. 6, 2008 and published on Sep. 12, 2008as Publication No. WO 2008/106958 A2, which claims priority to DEApplication No. 102007011337.6, filed Mar. 6, 2007, all of which arehereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention is directed to veneer ceramics for dental restorations inwhich the framework ceramic is made of yttrium-stabilized zirconiumdioxide.

2. Description of the Related Art

Yttrium-stabilized zirconium dioxide is a high-performance material ofextremely high strength that is used to an increasing extent inrestorative dentistry for framework ceramics for crowns, inlays, andbridges. The application of veneer ceramics is required for fineadjustment to the diversity of natural teeth. Up to the present, veneerceramics have presented a weak point in the ability of the restoredteeth to withstand stress.

Veneer ceramics should permit excellent shaping, be conformable to theadjacent teeth with respect to coloring, be highly resistant tochemicals, have a high flexural strength even after a directed heattreatment, and be characterized by outstanding adhesion to the frameworkceramic.

Powders or pastes are generally used as starting materials for producingthe veneer ceramics. The properties of the veneer ceramic are determinedby the chemical and crystallographic features as well as by the grainsize of the starting materials.

According to U.S. Pat. No. 4,798,536 A, leucite-containing dentalporcelains are produced by means of the fused glass. The leucite contentis in the range of 35 to 60 percent by weight. The high coefficient ofexpansion of the leucite-containing dental porcelain of 13 to 15×10⁻⁶/Kis used for veneering metal crowns. The flexural strength of the veneerceramic with leucite crystals is 80 MPa.

The use of lithium disilicate is proposed for a restorative toothprosthesis in U.S. Pat. No. 4,189,325 A. This reference concentrates onthe material system of Li₂O—CaO—Al₂O₃—SiO₂. The nucleating agents Nb₂O₅and Pt are added to promote crystallization.

U.S. Pat. No. 4,515,634 A suggests the addition of the nucleating agentP₂O₅ to the basic system of Li₂O—CaO—Al₂O₃—SiO₂ in order to improvenucleation and crystallization.

Laid Open Application DE 197 50 794 A1 describes the use of lithiumdisilicate glass ceramics use in the hot pressing method. However, ithas been shown that application of this method results in insufficientedge strength of the restored tooth and increased tool wear duringfinishing.

DE 103 36 913 A1 suggests a two-stage fabrication of the tooth to berestored. In the first step, lithium metasilicate is crystallized and ismechanically worked to form dental products. The lithium metasilic ateis converted to the stronger lithium disilic ate by a second heattreatment. Accordingly, the restored tooth is made entirely of glassceramic with lithium disilicate crystals.

German Patent DE 196 47 739 C2 describes a sinterable lithium disilicateglass ceramic and glass. The starting material is sintered to formblanks. These blanks are pressed at 700° C. to 1200° C. to form dentalproducts. The described lithium disilicate glass ceramic shows only aslight reaction to the adjacent casting investment during plasticdeformation.

EP 1 235 532 A1 describes a method for producing a high-strength ceramicdental prosthesis based on yttrium-stabilized zirconium dioxide. Theframework ceramics produced by this method have 4-point flexuralstrengths greater than 1200 MPa.

SUMMARY OF THE INVENTION

It is the object of the invention to make possible a translucent veneerceramic which has high flexural strength as well as excellent adhesionto the framework ceramic of yttrium-stabilized zirconium dioxide.

DESCRIPTION OF THE DRAWINGS

The invention will be described more fully in the following withreference to embodiment examples. The drawings show:

FIG. 1 an x-ray diffractogram (XRD) after the solid state reaction oflithium oxide and silicon dioxide (4 hours at 940° C.);

FIG. 2 a typical temperature curve for the production of the veneerceramic; and

FIG. 3 an XRD of a veneer ceramic according to the invention.

DETAILED DESCRIPTION

According to the invention, the objects are met in a veneer ceramic fordental restorations made of yttrium-stabilized zirconium dioxide whichis produced by the following components:

a) SiO₂ 58.0-74.0 percent by weight

b) Al₂O₃ 4.0-19.0 percent by weight

c) Li₂O 5.0-17.0 percent by weight

d) Na₂O 4.0-12.0 percent by weight

e) ZrO₂ 0.5-6.0 percent by weight

It can be advantageous when another nucleating agent, e.g., TiO₂, isadded within limits of 0.2 to 8.0 percent by weight in addition to thenucleating agent ZrO2.

The veneer ceramic is applied as powdered starting glass withcrystalline additions or without separate crystalline additions and issintered onto dental products of yttrium-stabilized zirconium dioxide bymeans of a defined temperature program in the range of 800° C. to 940°C. and crystallized in a controlled manner.

Surprisingly, it was shown that a very high adhesion strength to dentalproducts of yttrium-stabilized zirconium dioxide is achieved withspecific glass ceramics and a defined temperature program. The veneerceramic is translucent and has very good resistance to chemicals. Themain crystal phase of the glass ceramic comprises lithium disilicate.

Besides the powdered starting glasses of the glass ceramic, the veneerceramic can also contain powdered crystals as starting product. By meansof a defined heat treatment, the powdered veneer ceramic undergoes theprocesses of nucleation, sintering and fusion with theyttrium-stabilized zirconium dioxide and crystallization accompanied bythe formation of microcrystals.

Also, powdered lithium disilicate is preferably added to the startingglasses. The lithium disilicate can be produced by a solid statereaction.

The addition of TiO₂ promotes the process of nucleation andcrystallization of lithium disilicate. The veneer ceramic is thenadvantageously formed from a mixture containing the followingcomponents:

a) SiO₂ 58.0-72.0 percent by weight

b) Al₂O₃ 4.0-18.0 percent by weight

c) Li₂O 5.0-17.0 percent by weight

d) Na₂O 4.0-11.0 percent by weight

e) ZrO₂ 0.5-5.5 percent by weight

f) TiO₂ 0.2-8.0 percent by weight

Zirconium dioxide or a mixture of zirconium dioxide and titanium dioxideis used as nucleating agent for the controlled crystallization of theveneer ceramic based on lithium silicate materials. The addition oftitanium dioxide promotes the conversion of lithium metasilicate tolithium disilicate.

In a preferable veneer ceramic, lithium titanium oxide silicateLi₂TiOSiO₄, lithium aluminum silicate (beta spodumene) and small amountsof lithium metasilicate are crystallized in addition to the lithiumdisilicate.

The veneer ceramic can also be formed in such a way that the crystallineportion is below 40%. In this case, the veneer ceramic is thinlyapplied, serves for color matching, and imparts a particular aestheticgloss to the dental framework ceramic. The strength of the veneerceramic can be further increased by deliberate compressive stresses.

The oxides of elements Ce, Fe, Mn, Sn, V, Cr, In, and of rare earths Pr,Nd, Sm, Eu, Tb, Dy and Er can be used as coloring or fluorescingadditions.

To modify the technology, the additives La₂O₃, B₂O₃, P₂O₅, CaO, MgO, ZnOand fluoride can be added independently from one another inconcentrations of up to 4.0 percent by weight at most.

To produce the veneer ceramic, nucleation is carried out in thetemperature range of 500 ° C. to 680 ° C., and the melting andcrystallization is carried out in the temperature range of 800 ° C. to940 ° C. The nucleation and crystallization processes can be interruptedin that the veneer ceramic is cooled to room temperature betweennucleation and crystallization, stored, and then heated to thecrystallization temperature.

The adhesion strength between the yttrium-stabilized zirconium dioxideand the veneer ceramic is determined by flexural testing. For thispurpose, the powdered veneer ceramic is applied to the end face of tworound rods of zirconium dioxide and subjected to the defined heattreatment. The adhesion strength is determined by the three pointflexural test.

Veneer ceramics with an adherence to zirconium dioxide of at least 150MPa are preferred.

Twelve compositions are shown in Table 1 as embodiment examples of theveneer ceramics according to the invention.

TABLE 1 Examples 1 2 3 4 5 6 7 8 9 10 11 12 SiO₂ 71.0 71.1 62.0 70.569.7 61.2 70.5 70.5 69.6 69.6 58.9 60.5 Al₂O₃ 9.0 4.9 17.9 8.9 4.8 17.74.9 8.9 8.8 8.8 17.0 17.5 Li₂O 12.6 14.9 5.3 12.5 14.6 5.2 14.8 12.512.4 12.4 5.0 5.2 Na₂O 5.4 3.0 10.9 5.4 2.9 10.8 3.0 5.4 5.3 5.3 10.410.5 TiO₂ 5.0 4.9 1.2 5.0 5.0 2.5 ZrO₂ 2.0 1.1 3.9 2.0 1.1 3.9 1.1 2.02.0 2.0 3.7 3.8 CaF₂ 0.7 0.7 CaO 0.6 MgF₂ 0.7 BaF₂ 1.9 BaO 1.1 P₂O₅ 1.40.8 Total 100 100 100 100 100 100 100 100 100 100 100 100

The starting glasses were fused in platinum or platinum-rhodiumcrucibles at a temperature of 1530° C. and cast in water to produce afrit (FIG. 2).

To promote the controlled crystallization, the fritted starting glassesare tempered for approximately 4 hours at 580° C.±100° C. and powderedafter cooling. The grain size used ranges from 0.6 μm to 20 μm.

Powdered lithium disilicate can be added to the starting glasses. Thelithium disilicate is produced by a solid state reaction.

FIG. 1 shows the x-ray diffractogram (XRD) of the lithium disilicateproduced by the solid state reaction.

The moistened starting materials are applied to the dental frameworkceramic of yttrium-stabilized zirconium dioxide as veneer ceramic,melted at 890° C.±50° C. and crystallized in a controlled manner.

FIG. 2 shows the typical temperature curve during the production processfor the veneer ceramic.

All twelve examples of the veneer ceramics listed in Table 1 aretranslucent.

The optical effect and the mechanical resistance of the veneer ceramicsare influenced by the structure of the veneer ceramic as well as by theinteraction of the veneer ceramic and framework ceramic.

The coefficient of expansion (α) of the veneer ceramic and of theframework ceramic of yttrium-stabilized zirconium dioxide (TZ3Y) must beadapted to one another.

Based on the examples in Table 1, the coefficients of expansion (α) ofthe veneer ceramics are shown and compared with yttrium-stabilizedzirconium dioxide in Table 2.

TABLE 2 Examples 1 2 3 6 11 ZrO₂ α_(50-300°C.) × 10⁻⁶/K 8.9 8.7 8.9 8.98.7 9.6 α_(50-500°C.) × 10⁻⁶/K 9.8 9.8 9.8 9.6 9.3 9.8

The adhesion strength between the framework ceramic ofyttrium-stabilized zirconium dioxide and the veneer ceramic wasdetermined by the three point flexural test. For this purpose, thepowder of the veneer ceramic was applied between two cylindrical samplesof zirconium dioxide and subjected to a heat treatment corresponding toFIG. 2.

Table 3 shows the adhesion strength for selected samples, whereα=adhesion strength in MPa based on the three point flexural test andm=Weibull parameter.

TABLE 3 Examples 1 2 3 12 α MPa 162.1 183.1 173.4 172.4 m 7.6 13.2 3.54.4

Depending on the composition and heat treatment, the course ofnucleation and crystallization may differ in the veneer ceramics withhigh adhesion strength according to the invention.

Referring to Table 1 and a temperature of 890° C.±50° C., the examplesof veneer ceramics 1, 4, 8, 9 and 10 crystallize to lithium silicate andzirconium dioxide crystal phases. The zirconium dioxide serves as anucleating agent. The crystallization of the lithium silicate takesplace in two temporal stages. First, lithium metasilicate Li₂SiO₃ isformed and, through the subsequent reaction with the surroundingsilicate phase, lithium metasilicate is converted to lithium disilicateLi₂Si₂O₅.

Referring to Table 1 and a temperature of 890° C.±50° C., the examplesof veneer ceramics 2, 5 and 7 crystallize to crystal phases of lithiumdisilicate, beta spodumene, lithium titanium oxide silicate Li_(z)(TiO)(SiO₄), and lithium metasilicate. The crystallization of lithiumdisilicate Li₂Si₂O₅ is accelerated by the addition of titanium dioxide.FIG. 3 shows the XRD.

What is claimed is:
 1. Veneer ceramic for dental restorations comprisingyttrium-stabilized zirconium dioxide, characterized in that the veneerceramic comprises the following components: a) SiO₂ 58.0-74.0 percent byweight b) Al₂O₃ 4.0-19.0 percent by weight c) Li₂O 5.0-17.0 percent byweight d) Na₂O 5.3-12.0 percent by weight e) ZrO₂ 0.5-6.0 percent byweight, and comprises lithium disilicate as main crystal phase. 2.Veneer ceramic according to claim 1, characterized in that the veneerceramiccomprises the following additional components: a) La₂O₃ 1.0-4.0percent by weight b) B₂O₃ 0.0-2.0 percent by weight c) MgO 0.0-2.0percent by weight d) CaO 0.0-2.0 percent by weight e) ZnO 0.0-2.0percent by weight f) BaO 0.0-1.0 percent by weight g) P₂O₅ 0.0-2.0percent by weight h) fluoride 0.0-3.0 percent by weight.
 3. Veneerceramic according to claim 1, characterized in that the veneer ceramiccomprises oxides of elements Ce, Fe, Mn, Sn, V, Cr, In, and of rareearths Pr, Nd, Sm, Eu, Tb, Dy and Er, individually or in combination ascoloring or fluorescing additions.
 4. Veneer ceramic according to claim1, characterized in that the veneer ceramic comprises at least one otheralkali oxides for the suppression of crystallization of beta-quartzmixed crystals.
 5. Veneer ceramic according to claim 1, characterized inthat the Na₂O is contained for the suppression of crystallization ofbeta-quartz mixed crystals.
 6. Veneer ceramic for dental restorationscomprising yttrium-stabilized zirconium dioxide, characterized in thatthe veneer ceramic comprises the following components: a) SiO₂ 58.0-72.0percent by weight b) Al₂O₃ 4.0-18.0 percent by weight c) Li₂O 5.0-17.0percent by weight d) Na₂O 5.3-11.0 percent by weight e) ZrO₂ 0.5-5.5percent by weight f) TiO₂ 0.2-8.0 percent by weight, and compriseslithium disilicate as main crystal phase.
 7. Veneer ceramic according toclaim 6, characterized in that the veneer ceramic comprises thefollowing additional components: a) La₂O₃ 1.0-4.0 percent by weight b)B₂O₃ 0.0-2.0 percent by weight c) MgO 0.0-2.0 percent by weight d) CaO0.0-2.0 percent by weight e) ZnO 0.0-2.0 percent by weight f) BaO0.0-1.0 percent by weight g) P₂O₅ 0.0-2.0 percent by weight h) fluoride0.0-3.0 percent by weight.
 8. Veneer ceramic according to claim 7,characterized in that the veneer ceramic comprises oxides of elementsCe, Fe, Mn, Sn, V, Cr, In, and of rare earths Pr, Nd, Sm, Eu, Tb, Dy andEr, individually or in combination, as coloring or fluorescingadditions.
 9. Veneer ceramic according to claim 6, characterized in thatthe veneer ceramic comprises other alkali oxides for the suppression ofcrystallization of beta-quartz mixed crystals.
 10. Veneer ceramicaccording to claim 6, characterized in that the Na₂O is contained forthe suppression of crystallization of beta-quartz mixed crystals. 11.Veneer ceramic according to claim 1, characterized in that the veneerceramic comprises in addition to ZrO₂ also 0.2 to 0.8% by weight ofTiO₂.
 12. Veneer ceramic according to claim 1, characterized in that theveneer ceramic comprises at least one additive selected from La₂O₃,B₂O₃, P₂O₅, CaO, MgO, ZnO and fluoride in concentrations of up to 4.0%by weight at most.
 13. Veneer ceramic according to claim 6,characterized in that the veneer ceramic comprises at least one additiveselected from La₂O₃, B₂O₃, P₂O₅, CaO, MgO, ZnO and fluoride inconcentrations of up to 4.0% by weight at most.
 14. Veneer ceramics fordental restorations comprising yttrium-stabilized zirconium dioxide,characterized in that the veneer ceramic comprises the followingcomponents: a) SiO₂ 58.0-74.0% by weight b) Al₂O₃ 4.0-19.0% by weight c)Li₂O 5.0-17.0% by weight d) Na₂O 4.0-12.0% by weight e) ZrO₂ 0.5-6.0% byweight; and comprises lithium disilicate as main crystal phase, whereinan adhesion strength between the veneer ceramic and theyttrium-stabalized zirconium dioxide is at least 150 MPa.